Citrate gel synthesis of vanadium diboride

Abstract

Toward synthesizing vanadium diboride (VB2), mainly mechanochemical and carbothermic routes have been hitherto administired as well as combustion, self-propagating high temperature synthesis (SHS), and solid state metathesismethods. However, methods suggested to date do not deliberately deal with the oxidation state of the vanadium forming the oxide, formation conditions and morphology. Additionally, studies usually suggest an ultimate single reaction, disregarding side reactions. This narrow thermodynamic approach lacks key information in regards to the required conditions. Herein, this study reports the laboratory scale synthesis of VB2 nanopowders employing citrate gel method. The process is carried out using ammonium metavanadate (NH4VO3) as the vanadium source, boric acid (H3BO3) as the boron source, and citric acid (C6H8O7) as a chelating agent. Raw powder chelate is subjected to boro/carbothermic reactions under argon atmosphere in the range of 1100–1500 °C correlated with thermodynamic calculations with a 100 °C interval which are in good agreement with the experimental data. The resulting material is characterized to be at least 92% VB2 within the possibility of excessive graphitic carbon, carbon deficient vanadium carbide (V8C7) and/or non-stoichiometric VxBy(yx≤32) remaining as impurities. The formation mechanisms referring Liquid-Solid (LS) and Vapour-Solid (VS) reaction pathways throughout the major steps are described.